Printing operations undertaken in an offset printing press typically utilize lithographic printing plates. These lithographic printing plates are produced in a separate process involving the exposure of an image onto a plate substrate that typically comprises a thin aluminum alloy suitably treated so as to be sensitive to light or heat radiation.
One process of making a lithographic plate suitable for use on an offset printing press has been to employ a film mask. Exposing highly sensitive film media using a low power laser printer known as an “image-setter” typically produces this film mask. The film media is usually processed in some manner and is then placed in area contact with a photosensitive lithographic plate, which is in turn, “flood” or “area” exposed through the film mask. Such plates are referred to as “conventional printing plates”. The most common conventional printing plates used in such a process are sensitive to radiation in the ultraviolet region of the light spectrum. Typically, it is usually further necessary to amplify the difference between the exposed and un-exposed areas in a further chemical processing step that removes the unwanted coating and converts the plate into a lithographic printing surface ready for use on the press.
More recently, a method of exposing a lithographic printing plate directly through the use of a specialized printer known as a plate-setter has gained popularity. A plate-setter in combination with a computer system that receives and conditions image data for sending to the plate-setter is commonly known as a Computer-to-Plate or “CTP” system. CTP systems offer a substantial advantage over image-setters in that they eliminate the film mask and the associated process variation associated with that extra step. The CTP system receives the image data and formats it to make it suitable for outputting to an exposure head within the plate-setter. The exposure head in turn controls a radiation source, which is typically a laser, so as to image picture elements (pixels) on the lithographic plate according to the image data.
Lithographic printing plates imaged by CTP systems are typically referred to as “digital” printing plates. The radiation beams emitted by the exposure head induce a physical or chemical change to a coating on the digital plates. Currently, digital plates comprise either: 1) high-sensitivity photopolymer coatings (“visible light plates”) or 2) thermal photosensitive coatings (“thermal” plates). Visible light plates are typically exposed by a blue-violet laser diode of 10-100 mW. High power IR lasers in the range of 1 W to 100 W are used to expose thermal digital plates.
Like lithographic printing plates produced using film-based methods, many types of exposed or imaged digital printing plates typically undergo a further chemical processing step that removes the unwanted coating and converts the plate into a lithographic printing surface ready for use on the press.
Regardless of the method employed to image or expose a lithographic printing plate, the exposed printing plate is often pre-heated or pre-baked in an oven prior to being washed in a chemical solution during the subsequent chemical processing step. Additionally the processed printing plate can also be post-baked in another oven after the chemical wash step.
Once exposed or imaged, the printing plate typically undergoes the pre-heat step so as to render the image-wise exposed areas of the printing plate insoluble in the subsequent chemical development or processing steps. Hence, the un-exposed areas of the printing plate remain soluble and are washed away in the chemical baths to produce a final printing plate with the necessary differentiation between print areas and non-print areas. Typically, when the printing plates are exposed in a CTP plate-setter and then undergo this pre-heat step, the printing plates are referred to as “negative” or “negative-working” plates. Negative plates that are exposed with the use of conventional film masks are characterized such that the desired “printing image” will be exposed during the subsequent flood exposure. Likewise, negative plates that are imaged by a CTP system are characterized such that the desired “printing image” is imaged by the CTP plate-setter itself. In this context, the term “printing image” refers to the image that ultimately is printed on the press. In either case however, the printing image exposed on the printing plate is made insoluble by the pre-heat step such that it remains intact after the subsequent processing step. “Positive”, or “positive working” plates are essentially the opposite of negative plates. Accordingly, the background image or the non-printing image is directly exposed onto positive plates. Exposed positive plates typically do not undergo a pre-heat step. In fact, the exposed background images are rendered soluble upon exposure. Consequently, a positive plate can be chemically processed such that the exposed or imaged background is washed away to produce a final printing plate that comprises the necessary print image required on press.
Post-baking of a processed printing plate is usually conducted to impart specific characteristics to the printing plate. Such characteristics can include increasing plate life on press. Some plate manufactures claim that plate life can be increased as much as five fold. Different criteria can be used to determine when plate has reached its end-of-life. One such criteria suggests that a plate has reached its end-of-life when more than 25% of 200 lpi 1% dots imaged on the plate are worn off during printing (as determined visually). The benefits of post-baking are not limited to any one type of plate. Conventional and digital plates can be post-baked in accordance with their respective manufacturer's instructions.
Pre-heat and post-bake ovens have typically been conveyor ovens. Such an oven is disclosed in U.S. Pat. No. 6,323,462 (Strand).
Conveyor ovens typically need to be kept on all the time since their warm-up time is lengthy. Conveyor ovens are typically very large in size and thus require substantial space requirements. These space limitations are further exasperated when a processing line requires both pre-heat and post-bake capability. Consistent and uniform oven temperatures have a significant effect on the quality of the processed plate, thus further increasing the complexity of conveyor ovens which includes a myriad of blowers, heating elements and extensive ductwork. Ovens that comprise inductive heating (also known as RF heating) means or microwave heating means can offer instant warm up, but are expensive since they require many kilowatts of power at high frequencies.
Pressurized fluid bearings (also known as hydrostatic bearings) are well known in the art of tribology and have been used in processing equipment. U.S. Pat. No. 5,239,327 (Frank) describes the use of a plurality of hydrostatic bearings within a chemical processing tank. The hydrostatic bearings are submerged within a processing solution that is used to process a film web. In this patent, each of the bearings comprise a pair of juxtaposed housings on opposite sides of the film web. Each of the housings includes an aperture for emitting liquid under pressure so as to support the web without it physically contacting the housings. Frank discloses that the film web within the processor does not follow a linear path, and as such, in the curved potions of the web path, the juxtaposed face surfaces of the bearing houses are curved to define the necessary web path there-between.
A pressurized air bearing (also know n as an aerostatic bearing) is similar to any pressurized fluid bearing, except the fluid is air. Like hydrostatic bearings, pressurized air bearings have a porous or perforated plate, known as a bearing pad, through which pressurized air is pumped through and prevents contact between the pad and the moving object. The bearing pads can incorporate any air-permeable arrangement and include uniform and distinctly shaped openings or randomly formed openings created by sintered plates as an example. An air bearing can be single or double sided. In the later embodiment, the object glides between two parallel pads without touching either one and with practically no friction.
It has been shown in the prior art, that air bearings are capable of exhibiting exceptionally fast heat transfer to a planar object such as a printing plate. In regular convection ovens most of the heated air bypasses the printing plate, therefore heat transfer efficiency is low. In a heated air-bearing oven, most of the heated air can be forced to flow through a relatively small parallel gap between the printing plate and the bearing pads, thereby resulting in very good heat transfer. Another advantage is that such a heated air bearing oven is very small and has low thermal mass since there is no requirement to heat up a large enclosure.
However, one disadvantage of an air-bearing oven is that that the heat transfer efficiency is greatest when the planar surface of the printing plate is within a very small distance of the heated bearing pads. Consequently, it is desired to keep this distance, or alternatively, the gap between any two adjacent heating pads as small as possible to promote rapid heating. A plate however typically distorts upon heating and this distortion can cause the plate to contact a heating pad when such small distances and gaps are employed. This contact can cause damage to the plate, especially to its relatively delicate photopolymer or thermal photosensitive coating. Such damage would be highly undesirable as it would likely lead to on-press printing artifacts. This damage may be avoided by increasing the air bearing distance or gap, but at a cost of reduced heat transfer efficiency.
EP 0 864 944 A1 (Oelbrandt et al.) discloses an air bearing device that comprises two flat, planar air bearings plates used to heat an imaging element that can include various forms of paper, film, plastics, laminates and printing plates. It also discloses that the spacing between the two flat air bearing plates is in the range of 2 to 20 mm and that hot air is applied to both sides of imaging element within this spacing such that substantially equal flows at substantially equal air temperatures are created.
U.S. Pat. No. 5,181,329 (Devaney, Jr. et al.) discloses an apparatus for the drying of conventional film and paper during a photo processing operation. It also describes drying a web of paper or film between a pair of spaced, parallel air bearing members having flat surfaces defining a channel through which heated air is used to support the web. In addition to the air bearing air inlet holes, air bearing evacuation holes are provided at a predetermined distance from the inlet holes so as to maintain the heat transfer rate in the channel higher than the heat transfer rate in the web.
Clearly, there is a need for a simple oven capable of a rapid warm up. Further, such an oven should be compact and have a high thermal efficiency. Finally, such an oven should not contact the surface of the lithographic printing plate that is coated with a photopolymer or thermal photosensitive coating. Needless to say, any contact may result in damage to the exposed or imaged coating, ultimately resulting in undesired on-press printing artifacts.